Last week, with much excitement, the new London Maggie's Centre for cancer care opened on a sliver of land between the Fulham Palace Road and Charing Cross Hospital in Hammersmith. Already there are five centres in Scotland, designed by Page and Park, Richard Murphy, Frank Gehry and Zaha Hadid, but the London centre, designed by Richard Rogers's practice, is the first of six planned for England and Wales. A total of 13 centres are planned to be open by 2012.

They are the vision of Maggie Keswick Jencks who, while being treated for cancer, identified the need for patients to access emotional and psychological support and practical information in an uplifting environment, since, in her words, 'Above all, what matters is not to lose the joy of living in the fear of dying.' The buildings are closely linked to NHS oncology departments, but they don't have corridors or hospital paraphernalia. Visitors feel they are entering an 'open house' where the informality of conversation around a kitchen table helps to create an atmosphere of ease and sanctuary.

I was particularly excited to be offered the opportunity to create a series of gardens for the new centre, as I am a believer in the healing power of gardens and gardening. The calming effect of green and the fact that a beautiful, sensual environment can transport you can affect anyone. For those of us who choose to garden, there is nothing quite like the feeling of freedom that comes when you combine the cerebral with the physical. Building in the 'feel-good' is something that is now second nature in the way that I approach garden-making, but this was an opportunity to do it in the knowledge that all the energy was going to those who need it most.

The centre is based on the embrace of an arm that gradually envelops visitors as they move towards the kitchen at the heart of the building. A tall tomato-orange wall wraps around the building, providing protection

from the busy road and the hubbub of the hospital. My brief from Laura Lee and Marcia Blakenham, chief executive and vice chairman respectively of Maggie's, was that the building should be protected and cushioned by the green of a garden. The centre has been designed with three internal winter gardens and sun-filled roof terraces so that you are never more than a step away from the healing power of vegetation.

When I visited the site more than two years ago it was a delight to find that there was already a group of established London Planes there. They have textured trunks that lean at angles and immediately suggested a leafy woodland approach meandering between them. The entrance to the centre was key, as we knew that the biggest obstacle for many people scared by their situation was going to be getting over the threshold. The woodland walk has been planted with winter box (Sarcococca) for perfume and a soft undercurrent of leafy evergreen perennials, which will form a foil for a series of sensual ceramic sculptures by Hannah Bennett.

Hannah matched the glaze colours to the bark on the plane trees, and they lead your eye to the arrival courtyard, which is surrounded by a group of white Magnolia x loebneri 'Merrill'. There is a long bench here which connects the arrival courtyard through an opening in the front wall to the safety of the main inner courtyard. The last of the seven sculptures sits at the end of a leafy corridor by the front door, and is dished to hold a small pool of water to reflect the sky.

We surrounded the building with more than 100 Betula albosinensis var septentrionalis to filter noise and pollution. They are already showing the dark pink and mahogany patterning on the bark which, in time, will complement the warmth of the coloured walls. To play further with colour, a coppery layer of Luzula sylvatica 'Marginata' and orange-berried Iris foetidissima is massed beneath. The trees are concentrated in front of two frosted windows, which allow the shadow patterns to play on the glass and the visitor a shielded connection to the outside world.

Visitors are encouraged to find their own space inside the building, so there is a choice of courtyards to sit in. The north-facing courtyard is filled with the lush greenery of Tetrapanax, scented Nicotiana and the exotically perfumed climber Holboellia. The east courtyard has an outdoor fireplace, Sparrmannia and Iris japonica 'Ledger's Variety'. The focal point of the southern courtyard is a huge table under the shade of a feathery Albizia julibrissin, the Chinese Tree of Happiness, the bark and flowers of which are used as a calming sedative in Chinese medicine. There is perfume here, too, with Trachelospermum, honey spurge and scented pelargoniums. There will be grapes to eat from the vines that will cover the pergola, and at Cath Knox's suggestion we have planted some giant pots with peppermint and lemon verbena so that visitors will be able to pick their own herbs for a cup of tea and a chat.

Cath has been living with cancer for 12 years, and getting to know her has made the project very real for me. 'When you are told you have cancer, every moment counts,' she told me. 'To have those moments captured in the blossom of a magnolia that you may not see the following year, to stumble across perfume caught on the breeze or the scent of mint crushed between your fingers becomes incredibly meaningful.' I have always intuitively known that intimacy, sensuality and sanctuary in a garden are key to creating a sense of wellbeing, but it has been made so much more vivid seeing it through the eyes of someone who is seizing life with a new intensity. That intense connection with nature is something from which we can all benefit, and I feel privileged to have been able to create an environment which will offer that experience to those who may need it most.

· The London Maggie's Centre is open to the public as part of the Transport for London Open Garden Squares Weekend, 7-8 June (opensquares.org)